This present invention relates to an electrical controller for electric motors. In particular, the invention relates to a system for improving the control and operation of Alternating Current motors, wherein permanent magnets or steadily excited electromagnets move in the presence of stationary electrical windings. These motors will be referred to collectively as permanent magnet Alternating Current (PMAC) motors (although some may substitute steadily excited electromagnets for permanent magnets). PMAC motors may be found, for example, in high-speed hybrid turbochargers or other high-speed electrical devices.
Conventional PMAC motors utilise the movement of permanent magnets in the presence of stationary electrical windings. The stator windings must be excited by an oscillating or intermittent electrical current (i.e., AC or PWM) in order to exert an electromotive force upon the magnets as the magnets rotate or translate relative to the windings. Such motors are typically described as brushless alternating current permanent magnet motors or permanent magnet synchronous motors (PMSM). It is important to note that such motors are distinct from brushless direct current permanent magnet motors that have a different construction and control methodology.
Brushless AC permanent magnet motors are among the most mechanically simple, compact, and efficient types of motors. However, throughout the history of electric motors, practical embodiments have usually incorporated design features which compromise simplicity, compactness, and efficiency in order to impart favourable operating characteristics that simplify the task of controlling the motor. Examples of compromises include:                1. field weakening to limit speed from the inherent properties of the motor;        2. helical magnets to improve starting torque and predictability at low speeds;        3. electromagnets rather than permanent magnets to allow motor torque to be adjusted by the direct current signal powering the electromagnets;        4. a distribution of stator windings chosen in such a way as to smooth the torque output of the motor given a smooth alternating current (oscillating) input;        5. variable air gap (especially in ‘axial flux’ type motors) to allow the motor constant (relationship between current input and torque output) to be adjusted by mechanical means; and        6. the use of weaker magnets or passively excited (metallic) materials to reduce the sensitivity of the motor to the shape of the input alternating current signal.        
Electric motors usually operate at speeds below 200 Hz (e.g., electric cars=20 Hz to 100 Hz, automotive starter motors=30 Hz to 50 Hz, UK power station generators=50 Hz, typical pump motors=50 Hz, domestic appliances=10 Hz to 50 Hz, conveyors and pulleys=1 Hz to 50 Hz).
High-speed applications have typically favoured design approach number 7 from the above list: the use of passively excited materials. Examples include: J R Bumby, E Spooner, & M Jagiela, “Solid Rotor Induction Machnies for use in Electrically-Assisted Turbochargers”, Proceedings of the XVII International Conference on Electric Machines (ICEM), 2006; and S Calverly, “High-speed switched reluctance machine for automotive turbo-generators”, Mag. Soc. Seminar on Motors and Actuators for Automotive Applications, 2002.
Incorporation of the above design features significantly adds to the size, weight, cost and energy efficiency of motors. Additionally, for any chosen design, motor size, weight, and cost are typically proportional to torque output. Motors that operate at higher speeds can deliver equivalent power at lower torque, so equipment and transmissions are often specified to accommodate higher-speed motors where possible. However, higher speeds tend to exacerbate the challenges associated with motor control.
One application area of particular interest is turbomachinery. These devices, which operate upon gas at speeds approaching the sound barrier, spin at speeds in excess of 1,500 Hz. Turbomachines are well-known in aerospace and natural gas power generation, but they are increasingly found in automotive engines (turbochargers), industrial processes (compressors and heat recovery systems), domestic appliances (vacuum cleaners), and building heating and ventilation. The increasing popularity of turbo machines in comparison to fixed-displacement pumps and expansion chambers has created a further demand for high-speed motors and provides the prospect of reducing or eliminating the cost of high-ratio transmissions if higher-speed motors can be supplied. Of particular interest for the present invention is the electrification of the automotive turbocharger, which is the subject of an earlier patent (B Richards, “Turbocharger concept”, UK Patent no. 0624599.7, 2006).
Turbomachines require operating speeds in excess of 1,500 Hz, and some automotive applications require speeds above 2,500 Hz. Typical motor speeds below 200 Hz are not suitable for this application. Design compromises which use weaker magnets or passively magnetised materials can achieve speeds approaching 1,500 Hz, but these have relatively low power density. A typical 20 kW turbo compressor is approximately 15 cm×15 cm×10 cm in shape and requires approximately 1.6 Nm torque input in steady state. A typical passively excited electric motor operating at the same speed could supply the required torque from a sufficiently large motor. But such a motor would have a large rotor inertia. As the motor increases further in size to provide excess torque to overcome its own inertia during transient acceleration, the inertia of the motor increases in proportion to the extra torque produced, giving diminishing returns. By contrast, a motor with strong permanent magnets can achieve 10 times the torque from the same volume, allowing the motor to be smaller (on the order of 10 cm×10 cm×10 cm for 2 Nm) while still providing sufficient torque for acceleration. A problem of managing controller current remains.
Because of the design advantages described above and the emerging applications for high-speed machines, there has been an overall trend in the past three decades towards motors that are ever more difficult to control. This trend has coincided with, and depended upon, the widespread improvement of electronics and computers that enable ever more sophisticated control strategies.
Conventional brushless permanent magnet motors are generally either of the DC or AC type. Brushless DC motors accept ‘rough’ voltage input and smooth the flow of current internally by the inductance and resistance of the motor windings. Brushless AC motors (also called synchronous motors) require a smooth, sinusoidal (or near sinusoidal) current to be imparted by the controller. Neither is designed to accept ‘rough’ waveforms of current input.
The conventional approach to controlling a brushless permanent magnet motor is pulse width modulation (PWM). An example of this approach (relating specifically to PMAC) is shown in EP 2,159,909. This document utilises rapid PWM to simulate a smooth sine wave voltage input to the motor. This allows for precise control of position and smooth operation of the motor (especially at low speeds).
Brushless DC permanent magnet motors also use PWM to control the amplitude and phase of the motor's input voltage. The principle distinction between a brushless DC and a brushless AC permanent magnet machine is that a brushless AC motor additionally requires its PWM controller to synthesize a sinusoidal signal, while DC allows the PWM output to be a ‘rough’ voltage waveform. In either case (DC or AC), PWM generates a motor control signal of fixed amplitude and frequency and applies the signal at each commutation. The supply of the required signal, for example ‘rough’ (DC) or sinusoidal (AC), is achieved by varying the number and duration of pulses supplied to the motor. This generally entails providing several pulses per commutation of the motor to try to approximate the ideal waveform input required for the motor used. The overall amplitude (or voltage) supplied by PWM is therefore controlled by varying the number and duration of pulses supplied to the motor for any given phase.
However, use of a PWM controller is calculation intensive and necessitates the controller to operate at a frequency at least 10× (typically 100× or more) in excess of the rotational frequency of the motor. This means, for example, that an automotive turbocharger compressor would require a controller with at least 15,000 Hz internal operating frequency. This is well within reach for low-power logic circuits, but it approaches the limits of what can be achieved with high-powered electronic circuitry today.
A controller that may be embodied by the invention of the present application which will be described below produces ‘rough’ wave forms of current and requires (or corresponds to) a motor with an atypical design.
The proposed motor is of the brushless permanent magnet type, with properties that are different from either a typical brushless AC or brushless DC permanent magnet motor. The properties of this motor are known in the art, but this selection and combination of properties is unusual. Specifically, this motor has properties which enhance its ability to accept ‘rough’ waveforms of current (or, in the case of a generator, its tendency to produce ‘rough’ waveforms) and may be used in an advantageous way with the controllers embodying the present invention. Its properties are:                a. rotor magnets made of materials with strong permanent magnet properties and shaped in such a way as to have constant thickness through the angular dimension corresponding to shaft rotation and distributed about the rotor without gaps (all of which design features cause the electromagnetic field experienced by the windings near the magnets' edge to be similar in strength to the electromagnetic field experienced near the middle of the magnets);        b. a number of teeth (metal elements around which stator windings are wrapped) which is divisible by the number of electrical phase connections provided by the motor and a winding pattern such that any collection of series- or parallel-connected windings which constitutes a phase will everywhere be subjected to an identical (varying through shaft rotation but everywhere and at all times equivalent to each other) electromagnetic field arising from the motor magnets (so that all the windings in a phase are complementary and do not serve to counteract one another, at any rotational angle of the shaft);        c. a number of magnets (‘poles’) chosen such that b is implementable;        d. magnet angular thickness (arc length) and winding pitch (number of teeth spanned by a single winding loop) chosen so that the angle of rotation of the rotor through which an interface between adjacent magnets of opposing polarity passes within the span of a winding loop matches precisely the phase angle (proportion of the period of oscillation) through which the controller will maintain maximum current through that winding loop; and        e. minimum winding inductance achieved by a preference towards the individual wire loops forming the windings being connected in parallel rather than series to the extent allowed by other considerations such as the current and voltage specifications to which the motor must conform (in recognition of the fact that greater internal inductance will tend to smooth and delay the signals produced by the controller, with the extent of such transformation being a function of motor speed and thus difficult to accommodate within the controller).        
These features, although individually known in an academic sense, are not believed to be used in combination in typical, commercially-built electric motors, nor are the impact of these design features upon the controller widely considered or understood. In fact, the amalgamation within one organisation of both motor design and motor control is atypical within the industry. It will also be appreciated that this combination of features can be used to create a generator with unique characteristics that impact upon the design of the generator controller. In particular, the generator will provide an output that lends itself to DC rectification.